Alumina-based inclusions in molten steel are apt to be deposited (built up) on a refractory surface through physical contact with and/or chemical action on the molten steel. In this case, the deposit will grow and become large inclusions, and the large inclusions will be incorporated into slabs together with molten steel, causing slab defects and deterioration of slab quality. Moreover, if alumina-based inclusions in molten steel are deposited, for example, on an inner bore portion of a casting nozzle such as an immersion nozzle or an outlet portion of the casting nozzle having a great influence on a molten steel flow in a casting mold, thereby causing a change in initial shape of such a portion, it becomes unable to maintain a uniform flow of molten steel in the casting mold, and, due to the so-called “biased flow”, mold powder, gas bubbles and others are entrained into slabs, causing deterioration in slab quality. Thus, in casting, for example, of aluminum killed steel for thin sheets in which steel quality recently has become increasingly important as high grade steel, great efforts have been made to prevent adhesion of alumina-based non-metallic inclusions (hereinafter referred to simply as “alumina adhesion”) onto a refractory article such as a casting nozzle.
As a material for an alumina adhesion-resistant refractory product for use in a casting nozzle, there have been known a ZrO2—CaO—C based material, an SiO2—C based material, and a so-called “carbonless material” with minimal carbon. As the carbonless material, Al2O3 based, Al2O3—SiO2 based, SiO2 based and spinel based material have been commonly used, and a material with an enhanced ability of producing a compound having a melting point equal to or less than a molten steel temperature, such as a CaO—SiO2—ZrO2 based material, is also used recently. However, the commonly-used carbonless material has a problem that it can produce only a small amount of slag phase on a working surface of a casting nozzle through a contact reaction with alumina-based inclusions in aluminum killed steel, and, even if produced, a ratio of a liquid-phase to the entire slag phase (liquid-phase rate) at the level of a molten steel temperature is gradually lowered along with an increase in Al2O3 concentration in the slag phase due to continuous contact with molten steel, causing deterioration in alumina adhesion-resistant property, so that it will be sensitively influenced by steel grades and casting conditions such as casting speed, thereby leading to difficulty in obtaining stable alumina adhesion-resistant capability.
As an example of a technique of enhancing an ability of producing a compound having a melting point equal to or less than a molten steel temperature, the following Patent Document 1 discloses a refractory product of a carbonless material comprising CaO: 5 to 40 mass %, SiO2: 2 to 30 mass %, ZrO2: 35 to 80 mass %, and carbon: less than 5 mass % (including zero). However, according to the composition disclosed in the Patent Document 1, a slag phase containing ZrO2, as a low-melting-point compound, is produced at a working interface between the refractory product and molten steel by a contact reaction with alumina as inclusions in the molten steel, so that the ZrO2-containing slag phase becomes highly viscous, and alumina is more likely to adhere to the slag phase without flowing down, depending on a molten steel flow rate. Thus, there is a problem of failing to ensure stable alumina adhesion-resistant capability, by the influence of steel grades and casting conditions. Moreover, the carbonless material containing a large amount of CaO while reducing carbon to less than 5 mass % has another problem that a thermal expansion thereof is more likely to become greater than 1% at 1500° C. due to its strong ion-binding property, and strength becomes lower due to the low carbon content. Therefore, it is difficult to form a casting nozzle by using only the carbonless material. Thus, in many cases, the carbonless material is arranged in a region to be subjected to a contact with molten steel, and a Al2O3—C(AG) or ZrO2—C(ZG) based material having a thermal expansion of less than 1% at 1500° C. is used as a nozzle body material and integrated with the carbonless material, as described in the embodiments of the Patent Document 1. In this case, a problem still remains in terms of stability against cracking in a structural body during exposure to heat, due to a difference in thermal expansion between the two materials.
In regard to the aforementioned problem that stable alumina adhesion-resistant capability cannot be obtained due to variations in casting conditions and steel grades, it is tried to use a refractory product containing dolomite clinker (see, for example, the following Patent Document 2). In the dolomite clinker-containing refractory product, a CaO component in the refractory product and alumina-based inclusions in molten steel easily produce a liquid phase of a CaO—Al2O3—MgO based compound having excellent desulfurization ability, at an interface with respect to molten steel, to exert excellent anti-alumina adhesion effect. However, a material using dolomite clinker has a primary problem of poor handleability due to susceptibility to hydration (slaking problem).
Generally, dolomite clinker is a particular raw material in which a highly-active CaO component exists in a continuous matrix, and fine MgO crystal grains are dispersed in the matrix. Thus, while dolomite clinker has superb reactivity with alumina in molten steel and high alumina adhesion-resistant capability, it easily produces calcium hydroxide (Ca (OH)2) when CaO in the matrix contacts moisture in the air or contacts water directly (so-called “slaking”). If CaO-containing particles are hydrated, volume expansion due to Ca(OH)2 produced through hydration causes not only internal destruction of the particles but also destruction of the entire microstructure of the refractory product, thereby leading to difficulty in maintaining a shape as a structural body, in many cases. Therefore, various anti-slaking measures have heretofore been proposed.
Specifically, as means to prevent slaking of a CaO-based particle, there have been typically proposed (1) a technique of adding various additives into a CaO-based particle to coat CaO therewith, (2) a technique of carbonating a surface of a CaO particle, (3) a technique of coating a surface of a CaO particle with water-free oil, and (4) a technique of forming a hydration suppressive component layer between CaO-based particles.
The technique (1) includes a technique of incorporating one or more selected from the group consisting of Fe2O3, Cr2O3 and TiO2, into a CaO or CaO—MgO particle, in a total amount of 10 mass % or less, as described in the Patent Document 3. However, the technique based on the addition of an oxide other than CaO and MgO can improve slaking resistance but to an insufficient extent, and a low-melting-point substance such as 2CaO.Fe2O3 (melting point: 1447° C.) or 2CaO.Al2O3 (melting point: 1360° C.) is produced, which causes a problem of impairing refractoriness.
The following Patent Document 4 also proposes a refractory product for continuous casting, which contains: 1 to 97 weight % of CaO/TiO2 (mole ratio: 0.27 to 1.5) based clinker and/or CaO—TiO2—ZrO2 (predetermined mole ratio) based clinker; 3 to 40 weight % of a carbon raw material; and 96 weight % or less of other refractory raw material. However, slaking resistance is improved but to an insufficient extent. If the refractory product is prepared such that the above components are contained in particles in respective amounts enough to obtain sufficient slaking resistance, a low-melting-point substance is produced, which causes a problem of impairing refractoriness. Particularly, in the case where the clinker contains ZrO2, a problem of deterioration in an alumina adhesion-resistant effect will arise. Moreover, when the CaO-containing clinker is used in combination with Al2O3-based aggregate, a low-melting-point substance is produced at 1360° C. or more, so that refractoriness as a casting nozzle to be used at a temperature of 1500° C. or more will be deteriorated. In a casting nozzle generally formed using a plurality of materials, there is a problem of deterioration in flexibility of material arrangement, as with the former case where the one or more components easily reacting with CaO to produce a low-melting-point substance are dispersed in the entire clinker. Further, in the case of using the CaO-containing clinker, the refractory product exhibits a high expansion characteristic due to its strong ion-binding property. Thus, in a usage environment of a continuous casting nozzle to be subjected to rapid heating and rapid cooling, a problem still remains in terms of thermal shock resistance.
As to the technique (2), the following Non-Patent Document 1 reports that slaking resistance is improved by subjecting a CaO sintered body to a heating treatment under a CO2 atmosphere to form a CaCO3 film in a surface of the CaO sintered body, which is known as an anti-slaking technique for calcia clinker (lime clinker). However, in the technique (2) and the surface coating technique using oil (3) alike, during a kneading process in which CaO-containing particles each coated with a thin and soft film are mixed with refractory particles having the same level of hardness as a polishing material, the surface coating layer is easily peeled by mutual collision and shearing force of the particles, which causes a problem of loss of slaking resistance. Even if a thick film layer is formed to solve this problem, for example, by a carbonation treatment, a film defect occurs due to a difference in thermal expansion between the CaCO3 film and an interface of CaO in each particle, which causes a problem of deterioration in slaking resistance, despite the intention.
As to the technique (4), the following Patent Document 5 proposes techniques for a continuous casting nozzle prepared by subjecting a mixture comprising 40 to 90 wt % of lime, 10 to 60 wt % of carbon, and 0.1 to 10 wt % of one or more selected from the group consisting of boron carbide, boron nitride and boron, to kneading, shaping and burning. In the Patent Document 5, there is the following description: “although metals other than boron are also effective in preventing slaking of a lime-containing refractory product, boron carbide, boron nitride and boron exhibit extremely significant effects as compared with them.”, and it is assumed that the reason is because “boron or boron compound is transformed into B2O3 through compounding or decomposition during burning for a nozzle, and lime is coated with the B2O3”, and “the added boron carbide or boron nitride, or boron carbide transformed from the added boron through compounding with carbon, have properties similar to those of carbon, and thereby they are substituted for carbon and incorporated into lime as a solid solution to coat the lime”.
However, under a reducing atmosphere, boron carbide, boron nitride or boron has low reactivity as compared to oxides, so that it is not sufficiently effective in forming a film for coating a surface of a particle, such as a CaO surface, and hard to coat the surface of the particle, such as a CaO surface, without any defect. Thus, although some effect on hydration of CaO can be obtained by the technique disclosed in the Patent Document 5, the effect is significantly small. As above, this technique cannot provide a casting nozzle with handleability equivalent to that of a product formed using a non-hydratable component such as a conventional alumina-based component, so that it is still impossible to solve the technical problem of preventing hydration of CaO.
A second problem in the material using dolomite clinker is that it exhibits a high expansion characteristic. Such a high expansion characteristic is exhibited because a basic oxide such as CaO or MgO fundamentally has a strong ion-binding property. Excellent alumina adhesion-resistance can be ensured by arranging such a dolomite clinker-containing material to define an inner bore surface of a casting nozzle. On the other hand, when the high-expansion, dolomite clinker-containing material is used as an inner bore material and combined with a low-expansion nozzle body material, a resulting nozzle will always face a risk that breaking occurs due to a thermal expansion difference between the two materials. As measures against the risk, a technique for allowing such a type of nozzle to be stably used as a casting nozzle is disclosed, for example, in the following Patent Documents 6 and 7. However, the technique involves a production problem caused by complexity in production process and nozzle structure.